You can customize the buttons, menu buttons as well as the color of the checkboxes or radio buttons in your campaign.
Go to the 'Look & feel' step and click on 'Change Look and Feel'. This will open a workspace dedicated to the design of your campaign. Go to the 'Buttons' tab:
Use this section to set the style, format and color of the buttons and the menu buttons (top right of the iframe), as well as assign some colors to the radio buttons and checkboxes.
You can also change the button labels using the 'Button text' and 'Menu text' sections, located outside the campaign design workspace (still in the 'Look & Feel' step).